1. Technical Field
The present invention relates generally to lifting gear such as cranes which use high-strength fiber ropes instead of steel ropes. In this respect, the invention in particular relates to an apparatus for recognizing the discard state of a high-strength fiber rope in use at such lifting gear comprising a detection device for detecting at least one rope parameter as well as comprising an evaluation unit for evaluating the rope parameter and for providing a discard signal in dependence on the rope parameter evaluation.
2. Description of Related Art
In recent times, trials have been made with cranes to use high-strength fiber ropes made from synthetic fibers such as aramid fibers (HPMA), aramid/carbon fiber mixtures, high-modulus polyethylene fibers (HMPE) or poly(p-phenylene-2,6-benzobisoxazole) fibers (PBO) instead of the proven steel ropes used for many years. The advantage of such high-strength fiber ropes is their low weight. Such high-strength fiber ropes are considerably lighter than corresponding steel ropes at the same rope diameters and the same or higher tensile strength. In particular with high cranes with correspondingly large rope lengths, a greater weight saving is hereby achieved which enters into the dead-weight load of the crane and results in correspondingly higher payloads with an otherwise unchanged construction design of the crane.
A disadvantageous property of such high-strength fiber ropes is, however, their break behavior or their failure without substantial, longer preliminary signs. Whereas the wear is clearly visible in steel ropes and signals a failure over a longer period in advance, for example by the breakage of individual steel wires and a corresponding splaying which is easily noticed, high-strength fiber ropes show hardly any signs of excessive wear which would be easily perceivable for the eye and which would show themselves clearly over a longer period before the actual failure. In this respect, intelligent monitoring measures are required to recognize the discard state of high-strength fiber ropes in time.
An apparatus is known from DE 199 56 265 B4 for monitoring the operation of hoist winches on cranes which monitors the rope force of the hoist rope and the lever arm of the hoist rope on the rope winch and determines therefrom the load applications acting on the rope winch which are stored in a load spectrum counter. This load spectrum counter is integrated into the hoist winch to maintain the history of the hoist winch retraceably on a removal and reinstallation of the hoist winch. A load spectrum counter is furthermore known from EP 0 749 934 A2 which determines the load changes which occur, determines the rope force acting on the hoist winch at each load change, calculates the load spectrum therefrom and calculates and displays the remaining service life of the hoist winch while considering the so-called Wöhler curves.
Such monitoring measures of the hoist winch can, however, not really reliably give the remaining service life or the discard state of a high-strength fiber rope since the high-strength fiber ropes are subject to a variety of strains and impairments which influence the wear and which are independent of the winch strain, for instance e.g. the deflection and bending strains at pulley blocks, external blows and buffets on the rope, surface contamination of components contacting the rope, etc. On the other hand, inflexible service life standards for high-strength fiber ropes are practically incompatible with respect to economic utilization of the actual service life and observation of the required safety since the service life and wear of the high-strength fiber rope can fluctuate strongly in dependence on the conditions of use and on the external influences on the high-strength fiber rope.